Apparatus for reducing the size of particles



Sept. 22, 1964 B. N. HOFFSTROM 3,149,790

APPARATUS FOR REDUCING THE SIZE OF PARTICLES Original Filed May 24, 1960 3 Sheets -Sheet 1 INVEN'I OR 80 N. Hoffstrom BY ww w ATTORNEY5' Sept. 22, 1964 B. N. HOFFSTROM 3,149,790

APPARATUS FOR REDUCING THE SIZE OF PARTICLES Original Filed May 24. 1960 3 Sheets-Sheet 2 INVENTOR Bo N. Hoffstrom ATTORNEYS Sept. 22, 1964 B. N. HOFFSTROM 3,149,790

APPARATUS FOR REDUCING THE SIZE OF PARTICLES Original Filed May 24, 1960 3 Sheets-Sheet 3 INVENTOR B0 N. Hoffsfrom ATTORNEYS United States Patent 3,149,790 APPARATUS FOR REDUCING THE SIZE OF PARTICLES Bo Nilsson Holfstrom, Los Angeles, Calif, assignor to Douglas Aircraft Company, Inc., San Carlos, Calif. Continuation of application Ser. No. 31,401, May 24, 1960. This application Sept. 16, 1963, Ser. No. 308,932 8 Claims. (Cl. 24155) This application is a continuation of application Serial No. 31,401, filed May 24, 1960, now abandoned.

This invention relates to apparatus for reducing the size of particles and more particularly to such apparatus in which the particle size reduction is effected by impact.

At the present time particle size reduction (often called pulverization) is effected bycrushers, roller mills, ball mills, hammer mills, impact mills, disc mills and jet mills, each of which has certain advantages and disadvantages. Generally each type of apparatus is specifically developed for a particular application where its advantages outweigh its disadvantages. Accordingly there is no basic type of pulverizer which is in universal use.

For example, a ball mill is relatively eflicient for fine grinding. However for a given capacity, it is large and expensive and also suffers from the disadvantage of taking a long time to stabilize after start-up or change in feed rate. A conventional hammer mill or impact mill is also suitable for fine grinding or pulverization but the existing designs suffer from two disadvantages which reduce their efficiency. They create excessive air turbulence which absorbs a substantial part of the input energy without any corresponding advantage and they are generally designed to impart more than one impact to the material before it is discharged thus wasting energy on impacting particles which are already sufficiently reduced in size. The jet mill is considered most suitable for very fine grinding. Here the particles are accelerated by a very fast stream of gas or liquid and then made to impinge upon each other or upon a target. At its best, this apparatus is ineflicient and normally considerably more fluid is pumped than is necessary for optimum operation which further reduces the overall efiiciency of the jet mill.

The amount of reduction of particle size which is a measure of capacity of apparatus of this type is generally defined as the amount of new specific surface created, usually expressed in cm. per gram and there is a direct relationship between new surface and energy required for any given material. Thus for high efficiency, all input energy must be directed toward creating new surface and losses such as friction, fluid pumping and turbulence must be eliminated or minimized.

As the amount of size reduction or new surface goes up, intensity of energy input, i.e. horsepower required per ton per hour of material, goes up. At the same time factors causing loss of energy become more important.

With these considerations in mind, it is the primary pur ose and object of the present invention to provide improved apparatus for reducing the size of particles by impact with an efliciency considerably higher than is presently obtained with impact mills, hammer mills, or jet mills and which has the same or better efiiciency than a ball mill at the size and cost of only a fraction of that of the ball mills.

It is a further object of the present invention to provide improved impact mills which have the advantage of immediate stabilization, i.e. a change in feed is immediately followed by a corresponding change in output.

It is also an object of the present invention to provide improved impact mills which are of relatively simple and inexpensive construction and in which the parts sustaining the most severe wear are readily replaceable.

In attaining these and other objects, the present invention employs the basically known technique of accelerating the particles, which are to be ground, to a certain velocity and directing them onto a target. However by the utilization of unique apparatus these functions are accomplished with considerably higher elficiency than is presently obtained with impact mills, hammer mills or jet mills.

Additional objects and advantages will become apparent as the description proceeds in connection with the accompanying drawings in which:

FIGURE 1 is a side elevation of apparatus constructed in accordance with the present invention;

FIGURE 2 is a transverse section taken along line 22 of FIGURE 1;

FIGURE 3 is an enlarged fragmentary section of a portion of the apparatus of FIGURE 1; and

FIGURE 4 is an enlarged fragmentary view of the impact or target area.

Referring now in greater detail to the drawings, the principal components of the apparatus of the present invention include an essentially cylindrical housing 10 supported on a stand at 12, the housing 10 supporting an inlet funnel 14 at its upper surface and an electric drive motor 16 on its lower surface. The housing 10 carries a target ring 18 against which the particles are directed by a rotor assembly indicated generally at 20. The target ring, which is of relatively massive construction, is provided with a plurality of closely spaced notches 19 on one face of each of which a hardened steel target 23 is positioned for example by brazing. Each target thus has an impact face 21 which is preferably fiat. The finished product is withdrawn through a tangential outlet 22.

The housing includes a lower annular plate ring 25 welded to the stand 12 and detachably secured by screws 24 to a ring 26 welded to the undersurface of an intermediate ring 28, the outer portion of which forms the bottom wall of a chamber 30 through which the stream of fluid and particles passes. Detachably secured by screws 32 to the inner periphery of the ring 22 is a collar 34 upon which the drive motor 16 is removably mounted in axial alignment with the center of the rotor assembly 20.

The inner and outer walls of chamber 30 are formed by cylindrical members 36 and 38, respectively, welded to the bottom plate 28. A ring 40, the upper surface of which is in close clearance relation with the lower surface of the rotor assembly 20, is welded at its outer periphery to the cylindrical member 36 and at its inner periphery to a bearing support member 42 which is also welded to the lower plate 28. Welded to the upper edge of the outer cylindrical member 38 is a ring 44 to which is secured by screws 46 a ring 48 which forms the top wall of the chamber 30 and to which is welded a top plate 50 which forms the upper surface of the housing 10. The target ring 18 described in detail below is secured by screws 52 to the ring 48. A cylindrical collar 54 is welded at its lower end to the plate 50 and at its upper end carries a ring 56 to which an adaptor member 58 carrying the cover 14 is removably secured by screws 60.

The rotor assembly 20 comprises a main body member 62 and a top cover member 64, the two parts being secured together by screws 66 and 68 and being additionally held in the desired relative radial relation by dowel pins 70.

Formed in the main body member 62 of the rotor are four radially extending channels 72 which are symmetrically arranged about the axis of the rotor. The channels are open at their outer ends and at their inner ends are in open communication with an annular inlet throat 74 at the center of the rotor immediately below the inlet funnel 14. The walls of the channels are formed by identical wear strips 76 which are set in milled grooves 78 formed on the main body of the rotor. The strips 76 are received in the grooves 78 with a sliding fit and are held in place only by the top cover member 64 to facilitate their easy removal. Each of the strips 76 is bent on a constant radius about its center and is set symmetrically about the radius of the rotor. Thus centrifugal forces on the strips are completely balanced and taken as pure tension in the strips. The outer rim of the rotor is enclosed by four thin identical sheet metal strips 80 which are clamped in grooves formed in the rotor member 62 and the top cover plate 64. It will be understood that each of the strips 80 extends be tween the opposite ends of one of the wear strips 76 so as to leave the outer ends of the channels 76 unobstructed.

The rotor assembly is driven by an adapter collar 82 welded to its lower surface which in turn is connected by one or more pins 84 to a drive coupling 86 driven by the main motor 16. A suitable pre-loaded and sealed bearing construction indicated generally at 88 rotatably supports the collar 82 within the fixed cylindrical member 42. An upper bearing assembly 90 of similar construction is adjustably secured by a nut 92 to a downwardly projecting cylindrical section 94 of the member 58 and is received with a close sliding fit within an upwardly extending cylindrical projection 96 formed integrally with the rotor cover member 64 to provide bearing support for the upper end of the rotor.

It will be noted that by virtue of this construction, access to the rotor and the interior of the apparatus may be readily obtained by simply removing screws 46. This will permit removal of the intake funnel 14, the bearing assembly 90, the upper plates 50 and 48 and the target ring 18 to facilitate the replacement, adjustment or inspection of these parts.

Wear strips 76 which are expendable can be replaced merely by removing the screws 66 and 68 and the top cover plate 64. Because of the effect of the rotation of the rotor 20, only that portion of each of the wear strips which is at one side of the channels 72 receives any wear in operation. Since the wear strips are formed symmetrically, they need not be replaced after an initial period of wear but may simply be reversed and reinserted in the grooves 78 to present an unworn surface to the wear area. In any event, the relatively expensive rotor body never receives appreciable wear and thus has an extended service life.

Material fed through the funnel 14 will enter the rotor channels 72 if the rotor is revolving, for example in the direction indicated by the arrow 100 on FIGURE 2. The trailing wall formed by a portion of the strip 76 of each channel will give the particles a tangential velocity equal to the rotational velocity of the rotor. Due to centrifugal force acting on the particles, they will also be given a radial velocity outward through the channel. They will thus leave the channel at the rim of the rotor and enter a ballistic trajectory with a tangential velocity component v equal to the peripheral speed of the rotor and a radial velocity component u.

It can easily be shown mathematically that the radial component it equals the tangential component v under idealized conditions which are (1) that the acceleration of the particles start at the center and (2) that there is no friction between the particles and the channel wall. In this case the particle ballistic trajectory angle A after leaving the disc will be 45 and the resultant velocity v will equal v times the square root of 2.

In practice the idealized conditions are not fulfilled. The acceleration of the particles starts a certain distance r from the center and there is a certain coefficient of friction 1 between the trailing wall of the channel wall 72 and the particles. For reasons which will be explained, the channel Wall is not radial but its extension passes a certain distance e from the center of the where R is the rotor radius.

Applying this knowledge to a practical case, it has been found that the trajectory angle A tends to become about 40 instead of 45 as computed for idealized conditions. It will later become clear that an error of several degrees in the computation of A is insignificant.

Due to the friction between the particles and the wear strip 76, the resultant velocity along the trajectory v is less than it would be under frictionless conditions. This simply means that all the input energy has not been converted into kinetic energy of the particles. The loss of energy due to friction equals /2 the difference of the square of the theoretical v and the square of the actual v times particles mass flow. In practical cases it has been found that this energy loss is in the order of l015% of the theoretical particle kinetic energy after leaving the rotor.

Thus every particle entering the rotor will be given the same velocity and leave the rotor under the same trajectory angle A, both of which can be quite well established theoretically. Each and every particle thus contains the same specific energy, for example in terms of inch pounds per pound, and the total particle energy equals a -90% of input energy minus whatever losses there are besides particle friction loss.

To accomplish the size reduction which is the object of this invention, all that remains to do is to convert each particles kinetic energy into a force aimed at fracturing the particle. For this purpose the targets 23 are set with their striking surfaces 21 at identical angles B to the particle trajectories. For most efficient energy conversion the angle B should be In some cases it may be better, however, to use a smaller angle, for example to avoid clogging of the targets, if the material shows such tendencies. Another modification that would be advisable in some cases for similar reasons is to tilt the targets, a certain angle downward.

Upon impact the particle kinetic energy is distributed between the particle itself and the target in direct proportion to the deflection of the contact point relative to the particle on one hand and relative to the target on the other hand. Consequently, if the target is stifily backed by a mass that is very large compared to that of the particle and has a surface hardness that is very large compared to that of the particle, the deflection of the contact point relative to the target will be negligible and practically all the particle kinetic energy will be spent in deflecting, i.e. breaking up, the particle itself. Thus, target material may be steel that can be hardened, tungsten or boron carbide, or any other suitable material.

For perfect conditions, the target surface 21 should be curved to retain constant angle B to any trajectory that would intersect the surface. It is readily realized however that a deviation of a few degrees from the desired angle between the trajectory and the surface 21 would produce such a small change in velocity component normal to the surface, even under frictionless conditions, that it would be entirely insignificant from an energy point of view. Thus, for practical reasons the surfaces 21 are flat, and it is not important if the original determination of trajectory angle A should be off a few degrees, causing a corresponding error in determination of target angle B.

Consideration shall now be given to possible losses other than particle friction losses. Rotor bearing losses can be held very small and will be considered insignificant.

The only remaining loss is fluid pumping and friction loss. The analysis of this loss is the same whether the fluid is a gas or a liquid. In the following example, it is assumed to be air.

The air in the channels 72 is exposed to centrifugal forces just as the particles. It will, consequently, move radially outward if not prevented by restrictions. The more air that moves, the more input energy will be required to move it. Consequently the air flow must be restricted as much as possible. This can be done by restricting the number and size of the rotor channels 72 and/ or by restricting the size of the tangential outlet 22.

Possible restriction of the air movement is limited by the following considerations. The number of rotor channels 72 must be such that any particle entering the funnel 14 finds an immediate way out. This means that no surface on the rotor where particles might collect must be tangential, because the centrifugal force would hold the particles stationary relative to the rotor on such surfaces, and the effect could be clogging of passages. For this reason, the minimum practical number of radial channels in the rotor is three. Conditions are further improved if the number is increased to four as shown. Next, the size of each channel is determined by the maximum particle size that the mill is going to handle. The minimum crosssectional area of each channel must be large enough to accommodate the largest piece of material that may be expected. Thus, the minimum total air outlet area through the rotor is determined by these two factors.

The air flow could be limited by restricting the tangential outlet 22 if it were not for another requirement. If the air is allowed to pass freely through the rotor channels under the influence of centrifugal force, its peak radial velocity would become the same as the rotor rim velocity v or approximately the same as the particle radial velocity u. But if the air movement were restricted for example by reduction of the size of the'tangential outlet 22, its velocity through the channels 72 would become less than the particle velocity it. This would mean that the particles, while moving out through the rotor channels 72 would be subjected to an air drag opposing the particle movement, and the particle trajectory velocity would be reduced. The eifect would be more pronounced the smaller the particle, thus reducing efficiency of the milling of the fine particles more than that of the coarse particles. Thus the outlet 22 must be large enough to accommodate all the air that can pass through the minimum sections of the channels 72 with a velocity equal to v.

The peak velocity may develop at any radial point along the channels 72 or even in the inlet rotor section 74. The peak particle velocity, however, is reached only at the very exits of the channels 72. In order to avoid the generation of unwanted air drag at these points, the inlet funnel and rotor channels thus must be given a shape that produces peak air velocity at the channel exits. This is done simply by making these areas smaller than any other area along the air and particle passages including the outlet 22. For practical reasons, explained later, it is desirable that the channels 72 are symmetrical about a radius, thus the off-center location of the walls provided by the strips 76.

The minimum air pumping energy can now be determined. Theoretically, it equals /2 the mass flow of air,

'wAv

Where E =air pumping energy, H.P. -w=air spec weight, lb./in. A=total rotor exit area, in.

v=rotor rim velocity, in./ sec. g gravitational acceleration, in./sec.

In practice, air radial velocity never quite reaches rotor rim velocity, and pumping energy is slightly, say about 10%, less than given by this equation. The air pumping energy will be required whether the machine is doing any grinding or not.

The same is true for the air friction energy. This is caused by air turbulence set up by the rotor spinning in the stationary housing. It can be reduced only to a certain point by keeping the rotor outside as smooth and close to a circle as possible. For this reason, the gaps along the rotor periphery between the channel outlets are covered by the smooth walls 78 and the rotor top and bottom are smooth without unnecessary protrusions. With these measures, and also by holding the axial clearance between the housing walls 50 and 40 and the rotor top and bottom small, the air friction loss can be held to about a small fraction, say 10%, of the air pumping loss. Thus, E represents approximately the sum of air pumping and friction losses.

The fact that the air pumping and friction losses do not change as the flow of material through the mill changes is very significant. It has been shown that the minimum air pumping loss depends on factors that have nothing to do with the amount of grinding done by the machine, but by particle size in the feed and rim velocity of the rotor. In a practical case, for example, the rotor exit area is 6.25 in. rotor rim velocity 6500 in./sec., thus the air pumping and friction loss is 30 horsepower. This energy, then, must be provided even if no milling is done.

The energy required to accelerate, or pump, the particles can be calculated in the same manner as that required to pump the air, thus F22 6600g where:

E -zparticle pumping energy, H.P. F particle feed, lb./sec. v, g as before In order to treat one metric ton per hour of material or 0.61 lb./sec., in this particular case, then 10 horsepower is required. Thus, if a grinding capacity of 3 tons per hour is used, a total of 60 horsepower is required, half of which is not used for grinding but simply for pumping and heating the air. l015% of the remaining 30 horsepower is lost due to particle friction. It may thus be said that the total grinding efiiciency in this case is about 45%. If, instead, a total grinding capacity of 30 metric tons per hour would be utilized, the total power requirement would be 330 horsepower of which 30 horsepower still would be used for handling the air. Assuming 12% of the remaining energy would be lost in particle friction, total grinding efficiency then would be about 80%. If material input is again doubled, the efficiency would be further increased to about It is important to realize that the potential efficiency of this type of mill can be realized only when high production is maintained.

It can be shown that the limit in capacity, imposed by the ability of the passages to handle the flow of material through the machine is very large and does not become critical in normal cases. The capacity of a mill of the construction shown would be at least 200 tons per hour if the rotor diameter is 36 inches. The only modification necessary to handle this flow would be to add three additional tangential outlets similar to the one at 22.

All of the air movement has been treated as a loss. It actually does serve a practical purpose. The blast of air reaching the targets together with the material helps keep the targets clean. It also removes the ground material from the immediate neighborhood of the targets. The air flow further helps move the ground material out of the mill through the tangential outlet 22 and convey it to the next process, which may be a classification, or to storage.

An important feature of the present design is that once a particle has struck a target 23, the fragments are immediately carried out of the machine and will not be subjected to another impact unless they are fed back into the feed funnel 14. Thus, no energy is wasted on impacting particles which are already small enough. Assuming that the particles which are small enough are separated from those particles which are still too large in a classifier connected in series with the impact mill, then only the coarse fraction from the classifier would be returned to the mill for re-grinding.

Size reduction in each pass, defined as new surface created, can be taken as proportionate to rotor rim velocity squared. Thus a doubling of rim velocity means that new surface produced in each pass quadruples. Total output of finished product is controlled by rim velocity and by amount of re-circulation. The same result can be achieved either by high rim velocity and low recirculation or by the reverse combination. Rim velocity is limited by strength of the material in the rotor. It can be as high as 10,000 inches per second or more, which have been found sufficient to produce micron size particles, even of a hard to grind material, with only moderate recirculation.

From an efiiciency standpoint, it is usually better to use lower rim velocity and higher re-circulation since particle pumping power increases as rim velocity squared but air pumping loss increases as rim velocity cubed. It can be shown that overall eficiency improves as material feed per unit rotor channel discharge area increases.

Another favorable effect of relatively high recirculation is that heating per pass becomes less.

At a rim velocity of 6500 inches per second, for example, air temperature will rise about 50 F. Most materials to be ground have lower specific heat than air and will experience a temperature rise in the range of 60-80 F. In cases of unusually high specific heat, particle temperature rise per pass might be as low as 40 F., however.

If no cooling takes place between passes, however, there is no thermal benefit from high re-circulation since end temperature in this case is determined by total energy input into the final product, and this, in turn, is constant for any given final specific surface.

In many cases the shape of the particles after grinding is important. Due to its function, particles produced in the present mill exhibit freshly fractured surfaces and sharp edges. Since they are not exposed to rubbing or crowding as in a ball mill, they have the appearance of crushed stone, contrary to the pebble-like appearance of particles produced in the ball mill. The corresponding difference in characteristic of the final product is important in many cases, for example in Portland cement, where highly active surface is beneficial.

Although it has been assumed for simplicity in the present description that the mill operates in atmospheric air, it is easy to see that it lends itself well for operation in a closed system filled with any gas that might be desirable, e.g. for protection of the material to be ground. It can also be operated with any liquid that might be used to carry the feed, or it can be completely evacuated, which would eliminate the fluid pumping and friction losses. To accomplish feed out without the aid of a fluid carrier the target angle must be reduced and combined with tilt to leave enough momentum in the particles after impact to carry them out of the target area and into suitable conveying means.

On the other hand, the system might be pressurized to prevent evaporation or sublimation of the material to be ground. Finally, operation at other than room temperature might be desirable. Some materials become brittle and easier to grind at low temperature, others at elevated temperature. The present invention can be readily adapted for operation at any temperature compatible with the construction materials used.

The invention may be embodied in other specific forms Without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is:

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for reducing the size of particles comrising a rotor having a plurality of channels connecting an inlet section at the center of said rotor with the periphery of said rotor, said inlet section being open to the exterior of said apparatus to permit the inflow of air and particles thereto, said channels being of progressively decreasing cross-section from the inner end thereof to the outer end thereof, said rotor comprising a main body member and a top cover plate, said main body member having a plurality of open grooves extending along the opposite sides of said channels, strip members removably positioned in said grooves and forming the walls of said channels, said strip members being clamped between said main body members and said top cover plate, means for rotating said rotor whereby particles delivered to said inlet will be moved in an air stream through said channels by centrifugal force, annular target means surrounding the periphery of said rotor, said target means having a plurality of impact surfaces in the path of said particles issuing from said rotor channels, and means forming an outlet in communication with the region adjacent said target means, the broken particles being delivered in said air stream to said outlet.

2. Apparatus for reducing the size of particles comprising a substantially cylindrical chamber having a substantially vertical axis and having top and bottom walls, means providing an inlet for air and for said particles extending through said top wall axially of said cylindrical chamber, means providing a tangential outlet for said air and particles, a rotor mounted in said chamber coaxially thereof, said rotor having a central upwardly facing opening for receiving air and particles from said inlet, means defining a plurality of channels connecting said opening with the periphery of said rotor, said channels being of decreasing section as they approach the periphery of said rotor and providing exit throats at the periphery of said rotor, means extending through the bottom wall of said chamber for rotating said rotor whereby particles delivered to said opening will be moved in an air stream through said channels and said exit throats by centrifugal force, an annular target means surrounding said rotor within said chamber, said target means having a plurality of substantially flat, essentially vertical impact surfaces against which the particles issuing from said exit throats are thrown with sufiicient force to break them, the broken particles being delivered by said airstream through said tangential outlet.

3. Apparatus according to claim 2 wherein said means for rotating said rotor comprises a motor carried by said bottom wall of said chamber, a drive shaft connecting said motor and said rotor and a bearing mounted within said chamber rotatably supporting said shaft closely adjacent said rotor.

4. Apparatus for reducing the size of particles comprising a. substantially cylindrical chamber having top and bottom walls, means providing an inlet for air and for said particles extending through said top wall axially of said cylindrical chamber, means providing a tangential outlet for said air and said particles, a rotor mounted in said chamber coaxially thereof, said rotor having a central upwardly facing opening for receiving air and particles from said inlet, wall means defining a plurality of channels connecting said opening with the periphery of said rotor, said channels being of decreasing section as they approach the periphery of said rotor and providing exit throats at the periphery of said rotor, said wall means being formed symmetrically about the axis of said rotor to permit reversal thereof to present an unworn surface at the trailing edge of said channels, means for rotating said rotor whereby particles will be moved in an airstream through said channels and said exit throats by centrifugal force, and annular target means surrounding said rotor within said chamber, said target means having a plurality of impact surfaces against which the particles issuing from said exit throats are thrown with suflicient force to break them, the broken particles being delivered by said airstream through said tangential outlet.

5. Apparatus according to claim 4 wherein said means for rotating said rotor comprises a motor mounted beneath the bottom wall of said chamber, a drive shaft extending through said bottom wall of said chamber and connecting said motor and said rotor, and a bearing mounted within said chamber rotatably supporting said shaft closely adjacent said rotor.

6. Apparatus for reducing the size of particles comprising a rotor having a plurality of channels connecting an inlet section at the center of said rotor with the periphery of said rotor, said channels being of progressively decreasing cross section from the inner end thereof to the outer end thereof, said rotor comprising a main body member and a top cover plate, said main body member having a plurality of open grooves extending along the opposite sides of said channels, strip members removably positioned in said grooves and forming the Walls of said channels, said strip members being formed symmetrically and a portion of each strip forming the trailing Wall of one of said channels and another portion of said strip forming the leading wall of the adjacent channel, whereby when said strips are reversed end for end in said grooves, they present an unworn surface at the trailing edge of one of said channels, means for rotating said rotor whereby particles delivered to said inlet will be moved through said channels by centrifugal force, and annular target means surrounding the periphery of said rotor, said target means having a plurality of impact surfaces in the path of said particles issuing from said rotor channels.

7. Apparatus for reducing the size of particles comprising a substantially cylindrical chamber having a substantially vertical axis and substantially horizontal top and bottom walls, means providing an inlet for air and for said particles extending through said top wall axially of said cylindrical chamber, means providing a tangential outlet for said air and said particles, a rotor mounted in said chamber coaxially thereof, said rotor having an imperforate bottom wall and having a central upwardly facing opening for receiving air and particles from said inlet, wall means providing a plurality of channels connecting said upwardly facing opening with the periphery of said rotor, said channels being of decreasing section as they approach the periphery of said rotor and providing exit throats at the periphery of said rotor, a motor mounted beneath the bottom wall of said chamber and connecting said motor and said rotor, a bearing mounted within said chamber rotatably supporting said shaft closely adjacent to said rotor and beneath said imperforate bottom wall of said rotor whereby, upon rotation of said motor, said drive shaft and said rotor, particles will be moved in an air stream through said channels and said exit throats by centrifugal force, and annular target means surrounding said rotor within said chamber, said target means having a plurality of substantially fiat, essentially vertical impact surfaces against which the particles issuing from said exit throats are thrown with sufficient force to break them, the broken particles being delivered by gravity and by said air stream through said tangential outlet.

8. Apparatus for reducing the size of particles comprising a substantially cylindrical chamber having opposed essentially parallel end walls, means providing an inlet for air and for said particles extending through one of said end walls centrally of said cylindrical chamber, means providing a tangential outlet for said air and particles, a rotor having a central opening facing said inlet for receiving air and particles from said inlet, wall means defining a plurality of essentially straight radial channels connecting said central opening with the periphery of said rotor, said channels being of decreasing section as they approach the periphery of said rotor and providing exit throats at the periphery of said rotor, said wall means being formed symmetrically about the axis of said rotor to permit reversal thereof to present an unworn surface at the trailing edge of said channels, means extending through the other wall of said chamber for rotating said rotor whereby particles delivered to said opening will be moved in an airstream through said channels and exit throats by centrifugal force, annular target means surrounding said rotor within said chamber, said target means having a plurality of substantially flat impact surfaces essentially normal to the path of said particles against which the particles issuing from said exit throats are thrown with sufficient force to break them, the axial length of said cylindrical chamber being substantially greater than the height of said exit throats to provide a chamber offset axially of said throats for the reception of said broken particles, the broken particles being delivered by said air stream through said tangential outlet.

No references cited. 

1. APPARATUS FOR REDUCING THE SIZE OF PARTICLES COMPRISING A ROTOR HAVING A PLURALITY OF CHANNELS CONNECTING AN INLET SECTION AT THE CENTER OF SAID ROTOR WITH THE PERIPHERY OF SAID ROTOR, SAID INLET SECTION BEING OPEN TO THE EXTERIOR OF SAID APPARATUS TO PERMIT THE INFLOW OF AIR AND PARTICLES THERETO, SAID CHANNELS BEING OF PROGRESSIVELY DECREASING CROSS-SECTION FROM THE INNER END THEREOF TO THE OUTER END THEREOF, SAID ROTOR COMPRISING A MAIN BODY MEMBER AND A TOP COVER PLATE, SAID MAIN BODY MEMBER HAVING A PLURALITY OF OPEN GROOVES EXTENDING ALONG THE OPPOSITE SIDES OF SAID CHANNELS, STRIP MEMBERS REMOVABLY POSITIONED IN SAID GROOVES AND FORMING THE WALLS OF SAID CHANNELS, SAID STRIP MEMBERS BEING CLAMPED BETWEEN SAID MAIN BODY MEMBERS AND SAID TOP COVER PLATE, MEANS FOR ROTATING SAID ROTOR WHEREBY PARTICLES DELIVERED TO SAID INLET WILL BE MOVED IN AN AIR STREAM THROUGH SAID CHANNELS BY CENTRIFUGAL FORCE, ANNULAR TARGET MEANS SURROUNDING 